The present disclosure relates to an optical disk manufacturing method and, in particular to an optical disk manufacturing method, which may be applied for a multi-layer optical disk having two or more information recording layers.
In the optical disk field in recent years, there has been a demand for increases in capacity for information recording. In order to increase the capacity for information recording, a multi-layer optical disk having multiple information recording layers is considered promising (see JP-A-2003-91868). Especially, an optical disk having two information recording layers (which is referred to as a two-layer optical disk, hereinafter) is in actual use and offers promise.
A high-density optical disk has a recording capacity of about 25 Gbyte in one-sided and single-layer. The high density optical disk allows an increase in capacity for information recording by adopting a wavelength of the light source as long as 405 nm and a numerical aperture (NA) of the objective as large as 0.85, for example, for a small spot diameter of the reading beam.
A high density two-layer optical disk has a recording capacity of 50 Gbyte in one-sided two layers. The high density two-layer optical disk has L0 and L1 layers. The L0 layer is an information recording layer, which is a reference layer, at a depth of 0.1 mm (100 μm) in the direction of incidence of laser light. The L1 layer is an additional information recording layer at a depth of 75 μm.
In order to form a substrate first in a method for manufacturing a two-layer optical disk, pits and projections of the L0 layer are formed on one main surface thereof, and a middle layer of an ultraviolet curing resin is formed on the L0 layer. The L1 layer is formed on the middle layer.
Emboss Process and 2P (Photo Polymerization) Process have been proposed as methods for forming the L1 layer. The Emboss Process and 2P Process are described below.
[Emboss Process]
A Pressure Sensitive Adhesive (PSA) film or a UV-curable PSA film is laminated on the first layer of a substrate, and a stamper is brought into intimate contact thereon and is pressurized to transfer. In some cases, UV rays may be irradiated from the substrate or stamper side, and both, in intimate contact, may then be UV-cured. After that, the stamper is peeled off from the substrate, and the pits and projections of the L1 layer are thus formed.
[2P Process]
A substrate and a stamper are faced against each other, and an ultraviolet curing resin is filled between the first layer of the substrate and the stamper. Then, under this state, they are UV-cured. After that, the stamper is peeled off from the substrate, and the pits and projections of the L1 layer are thus formed.
However, the Emboss Process and the P2 Process have problems as described below.
[Emboss Process]
The material costs of the PSA film or UV-curable PSA film are high, and the storage of the materials needs maintenance in order to prevent the distortion of the materials. The PSA film or UV-curable PSA film is formed through steps of uniform thickness coating on a peeled film and solvent uniform drying, for example, which may increase the constraints for materials and also increase the scale of manufacturing. The compatibility of the second layer between hardness as a transfer layer for holding transfer signals and the adhesiveness to the substrate is difficult.
Furthermore, as shown in FIG. 8A, a film 102 may be bonded on a substrate 101 out of place. A projection 111 due to the displacement in bonding has adverse effects as described below:
(a) the projection 111 causes the resulting optical disk to fall out of spec;
(b) the resulting optical disk may be easily deformed;
(c) problems may occur in subsequent manufacturing processes; and
(d) the resulting disk in the final form may peel because the overhanging part can be easily caught.
A recess 112 due to the displacement in bonding has adverse effects as described below:
(a) the transferability of the outer circumference is not enough;
(b) the servo becomes instable; and
(c) the corrosion of the upper and lower recording films may be easily induced or accelerated.
Furthermore, the adoption of a method in which the film diameter and the recording area, which is an area having the L0 layer and L1 layer, are reduced in order to prevent the displacement in bonding and the outer circumference of the disk is not used, contradicts the increase in capacity, which may sacrifice a desired recording density.
[2P Process]
As shown in FIG. 8B, since an ultrasonic curing resin 103 to be coated over the substrate 101 is liquid, it is difficult to form and maintain a uniform thickness before UV curing, which may cause unevenness in thickness. The shrinkage by UV-curing may cause a large skew of the substrate 101. The amount of ultraviolet curing resin filled between a substrate and a stamper is difficult to control. An excessive amount thereof may cause an outer burr 113 easily while an insufficient amount thereof may catch air bubbles easily and may cause a bubble caught part 114. It is difficult to have the compatibility of the second layer between the hardness as the transfer layer for holding transfer signals and the adhesiveness to the substrate.
Accordingly, an optical disk manufacturing method, which has good transferability and substrate adhesiveness, can reduce the skew of the optical recording medium and can manufacture a high quality optical disk is desired.